38o WILSON. [Vol. VI. 



stomodaeum, is in the median ventral line posterior to the pro- 

 totroch. The anus is not yet formed, but its future position is 

 indicated by a pigmented area that lies nearly in the centre of 

 the lower hemisphere. At this point, as will be shown further 

 on, the blastopore has closed, and here the mesoblast-bands 

 reach the surface. The subsequent elongation of the body 

 (PL XX) takes place in the vertical axis of the trochophore, 

 which coincides with the principal axis of the ovum. 



This preliminary statement will allow the following descrip- 

 tion to be made more condensed, and, I trust, more intelligible. 

 It may be added that the spiral period of cleavage in Nereis is 

 marvellously similar to the cleavage of the polyclade ovum as 

 described especially by Gotte, Selenka, and Lang, and shows an 

 equally striking similarity to the cleavage of the molluscan 

 ovum (Bobretzky, Blochmann, Conklin, etc.). It is important to 

 notice, however, that although thtfonn of cleavage in the spiral 

 period of Nereis is identical with that of the polyclade, yet the 

 corresponding blastomeres are only in part homologous. The 

 significance of this very remarkable fact is discussed further on.^ 



1 It is absolutely necessary to fix, at the outset, the terms employed in the orienta- 

 tion of the larva, since there is room for a wide difference of opinion which may 

 easily lead to a confusion of terms, and the orientation I shall employ differs radi- 

 cally from that of v. Wistinghausen. I shall regard the principal or vertical axis of 

 the trochophore as representing, roughly speaking, the dorso-ventral axis (antero- 

 posterior, V. W.), the prototrochal plane as approximately longitudinal (dorso-ven- 

 tral, V. W.), the gap in the prototroch as approximately posterior (median-dorsal, 

 v. W.), and the opposite point as anterior (median ventral, v. W.). My orientation 

 agrees essentially with that of Gotte and Lang as applied to the polyclade larva, while 

 V. Wistinghausen follows that of Selenka. The difference is essentially this : v. Wist- 

 inghausen orients the ovum and the trochophore directly with reference to the adult 

 body, and thus identifies the principal or vertical axis of the ovum with the antero- 

 posterior axis of the adult. I believe, on the other hand, that a shifting of the axes 

 takes place, so that, broadly speaking, the larval axes are at right angles to those of 

 the adult. Thus, (i) the posterior extremity, (2) the ventral region, and (3) the 

 anterior region of the embryo and early larva become in v. Wistinghausen's termi- 

 nology respectively (i) dorsal, (2) posterior, and (3) ventral. I regret this confusion 

 of terminology, since I am perfectly in accord with v. Wistinghausen in regard to the 

 facts. I wish, however, expressly to disclaim any intention of attaching a profound 

 morphological significance to the terminology employed; for, as will be shown here- 

 after, the axes of the young embryo cannot be very accurately defined. The general 

 axial shifting that takes place in the metamorphosis is, however, not ideal, but real — 

 it is an actual transference of material; and if this shifting is not recognized, the 

 terminology of the early stages becomes in the highest degree confusing and contra- 

 dictory. My orientation is, therefore, adopted for purely practical reasons, i.e. for 

 convenience of description. 



